Research and development on magneto-optical disks have been intensified as being rewritable optical disks, and some of the magneto-optical disks have been already practically used as external memories designed for computers.
In the magneto-optical disk, a magnetic thin film with perpendicular magnetization is used as a recording medium, and light is used in recording and reproducing. Thus, compared with a floppy disk or a hard disk including a magnetic thin film with in-plane magnetization, the magneto-optical disk has a larger storage capacity.
Recently, a still larger storage capacity is demanded, and in order to meet the demand, not only for the hard disks but also for the magneto-optical disks, earnest research has been made in pursuit of still higher recording density.
For the magneto-optical disk, the recording density is determined by the size of a spot of a recording-use or reproduction-use light beam formed on the disk. Further, the size of a recording bit to be reproduced is determined by a light beam diameter.
In the case of a normal optical recording, since a light beam is converged to a diffraction limit by a converging lens, the light intensity distribution shows a Gaussian distribution, and thus the temperature distribution on the magneto-optical disk is like a Gaussian distribution. Thus, an area having a temperature rise above a predetermined temperature has a smaller diameter than the light beam diameter. Here, if only the area having the temperature rise can be made subjected to reproduction, the recording density can be significantly improved.
Here, a method for reproducing a recording bit recorded at high density in which the above restriction can be avoided has been proposed (see, for example, Journal by the Japan Applied Magnetic Society, page 838, Vol. 15, No. 5, 1991).
As shown in FIG. 11, a magneto-optical disk which is composed of a substrate 21 whereon a readout layer 22 and a recording layer 23 are formed may be used in the above method. The recording layer 23 has large coercive force at room temperature. On the other hand, the readout layer 22 has small coercive force at room temperature. As the temperature of an area subjected to reproduction in the readout layer 22 is raised, the magnetization in the area is arranged in the magnetization direction in the recoding layer 23 by being affected by the recording layer 23. Namely, by the exchange coupling force exerted between the readout layer 22 and the recording layer 23, the magnetization direction in the recording layer 23 is copied to the readout layer 22.
In the above arrangement, a recording operation is carried out by the general magneto-optical recording method. In reproducing the recording bit, first, the readout layer 22 is initialized. Namely, in order to arrange the magnetization direction in the readout layer 22 in a predetermined direction (upward in FIG. 11), subsidiary magnetic field is applied from a subsidiary magnetic field generation device 26. Next, a reproduction-use light beam 25 is converged on the readout layer 22 by the objective lens 24 so as to raise the temperature at the portion, and as a result, the information in the form of a magnetization direction recorded on the recording layer 23 is copied to the readout layer 22. In the above method, only the area having a temperature rise above a predetermined temperature which is located around the center of a spot of the reproduction-use light beam 25 can be made subjected to reproduction, thereby enabling a reproduction of a smaller recording bit compared with the case where the conventional reproducing method is used.
However, in the above arrangement, prior to the reproducing operation, subsidiary magnetic field must be applied from the subsidiary magnetic field generation device 26. Moreover, in reproducing, the information in the form of a magnetization direction copied from the recording layer 23 to the readout layer 22 remains even after the temperature of the previously reproduced portion drops. Therefore, when the spot of the reproduction-use light beam 25 is shifted so as to reproduce the next recording bit, the previously reproduced recording bit (the bit just copied) still remains within the spot of the reproduction-use light beam 25. Since this will be the cause of generating noise in reproducing, the above method presents the problem in that an improvement in the recording density is restrained.